Method of forming a helical ball screw member

ABSTRACT

A method of forming a ball screw member from an elongated workpiece having an outer cylindrical surface, as well as the ball screw member formed by the method. A helical ball screw thread is machined on the outer cylindrical surface, the helical ball screw thread having a helical groove and a helical crest surface. An end journal or other end treatment is formed in the workpiece, the end journal or other end treatment being located during the forming operation relative to the helical crest surface. Alternatively the end journal or other end treatment is formed in the workpiece and the helical screw thread is then machined while the outer cylindrical surface is located relative to the end journal or other end treatment during the machining of the helical ball screw thread on the outer cylindrical surface. The outer cylindrical surface of the workpiece may be centerless ground prior to the machining of the helical ball screw thread. The helical crest surface may be ground after the machining of the helical ball screw thread to provide increased concentricity between components of the ball screw member. A high precision ball screw member may be formed from the ball screw member by performing a regrinding operation on the helical groove, the regrinding operation being positioned relative to the helical crest surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my co-pending applicationSer. No. 06/578,379 which was filed on Feb. 8, 1984, now U.S. Pat. No.4,638,548, issued Jan. 27, 1987.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to external helical ball screw membersand more particularly to an inexpensive method of making an externalhelical ball screw thread and associated end treatments on a workpiece.

2. Description of the Prior Art

Ball screw assemblies are in common use today on a variety of devices.Typically, a ball screw assembly consists of a ball screw member havingan external helical ball nut thread, a nut having an internal helicalball screw thread fitted over the ball screw member and a plurality ofbearing balls disposed partially within the external helical ball nutand the internal helical ball screw. The ball screw member may include avariety of end treatments, such as, but not limited to, an end journal,a transverse bore, and a screw thread, for positioning the ball screwmember relative to other machine elements and for interconnecting theball screw member with other machine elements.

Ball screw assemblies are often used because of their characteristics ofproviding a good load carrying capacity, both longitudinally andtransversely of the ball screw member, low frictional resistance todisplacement of the ball screw member relative to the nut, accuratetranslational positioning of the ball screw member relative to the nut,and accurate axial positioning of the ball screw member relative to thenut as the members are relatively displaced longitudinally. Therefore,the ball screw assemblies are often used in machinery requiring accuratetranslational positioning between various machine elements, particularlywhen extreme loads are experienced.

Numerous examples of devices using ball screw assemblies may be found inthe prior art. For example, U.S. Pat. No. 2,924,265 to J. Himka, issuedFeb. 9, 1960, uses a ball screw assembly and a device for adjusting avehicle seat. U.S. Pat. No. 2,930,252 to R. E. Sears et al., issued Mar.29, 1960, discloses a valve control using a ball screw assembly. U.S.Pat. No. 2,935,893 issued to E. Mazur on May 10, 1960 and U.S. Pat. No.1,967,482 issued to B. F. Schmidt on July 24, 1934 each show how a ballscrew assembly may be used in a steering gear. U.S. Pat. No. 3,159,046to J. L. Harned et al., issued Dec. 1, 1964 shows the use of a ballscrew assembly in a differential. An aircraft trim control using a ballscrew assembly is taught in U.S. Pat. No. 2,772,841, issued Dec. 4, 1956to D. H. Bonsteel. A ball screw assembly is used in various liftingjacks, as exemplified by U.S. Pat. No. 28,613, issued June 5, 1860 to C.F. Spencer. Ball screw assemblies are also typically used in worm geardrives, as disclosed in U.S. Pat. No. 3,672,239, issued to G. Titt onJune 27, 1972.

Another common use for ball screw assemblies is in drive mechanisms,tool holders and drill feeds for various machine tools, as exemplifiedby U.S. Pat. No. 2,957,368 to J. Hendrickson, issued Oct. 25, 1960; U.S.Pat. No. 3,640,147 issued to G. Fantoni on Feb. 8, 1972, and U.S. Pat.No. 2,375,991 issued to H. S. Hoffar on May 15, 1945.

Numerous other uses for ball screw assemblies are well known in the art.

Two methods have been used in the past for forming external helical ballscrew threads on the ball screw members of ball screw assemblies. Thetwo methods differ substantially in cost and result in external helicalball screw threads and ball screw members of substantially differingaccuracy.

The first method of the prior art is used to produce commercial orindustrial quality ball screw members of moderate precision. The firstmethod involves performing a rolling operation on the externalcylindrical surface of a workpiece in order to form an external helicalball screw thread on the cylindrical surface thereof. The externalhelical ball screw thread produced by the rolling operation defines anirregular surface which is of little value for locating the endtreatments for further operations. Therefore, all further operations onend treatments of the ball screw member formed according to the firstmethod are typically located relative to the pitch diameter of theexternal helical ball screw thread. Subsequently, the workpiece ishardened. While the rolling operation is inexpensive, it does not resultin a very accurate external helical ball screw thread. The externalhelical ball screw thread produced by the first method of the prior artis of moderate precision due to the difficulty of maintainingstraightness and lead angle accuracy during a rolling operation. Thelocation of end treatments relative to the external helical ball screwthread is of moderate precision as a result of difficulties arising fromthe angular disposition, due to lead angle accuracy, of locating pinswhich are used to locate the end treatments relative to the pitchdiameter of the external helical ball screw thread. Furthermore, theprocess of positioning the end treatments using locating pinssubstantially increases the difficulty of forming the end treatments.

The second method of the prior art is used for making high precisionball screw members. The second method involves performing a rollingoperation on the cylindrical surface of the ball screw member to form anexternal helical ball screw thread, forming, by suitable means, the endjournal or other end treatment, heat treating the workpiece, and thenregrinding the external helical ball screw thread to more accuratedimensions in relation to the end journal or other end treatment of theball screw member. While this results in a far more accuratelydimensioned and positioned ball screw assembly than the first method ofthe prior art, described above, it is also substantially more expensiveto produce a ball screw member according to the second method than it isto produce one according to the first method.

What is needed, therefore, is a new method for producing commercial orindustrial quality ball screw members which avoids the difficultiesassociated with the first method of the prior art with locating the endtreatments of the ball screw member relative to the external helicalball screw thread. The new method preferably should be easier to carryout and to automate and yet result in a ball screw member of greateraccuracy than that produced by the first method of the prior art. Formany commercial purposes, a ball screw member is needed having anaccuracy intermediate those provided by the first and second methods ofthe prior art. Accordingly, what is also needed is an inexpensive methodfor forming an external helical ball screw in a ball screw member, whichmethod produces an external helical ball screw thread in a workpiece ofintermediate accuracy between the above described methods at a costsubstantially less than that required to produce an external helicalball screw according to the second above described method of the priorart.

SUMMARY OF THE PRESENT INVENTION

The present invention provides an inexpensive method for forming anexternal helical ball screw thread in a workpiece, the external helicalball screw thread being of an intermediate accuracy, relative toprevious methods for forming external helical ball screws. The method ofthe present invention avoids the difficulty of using locating pinsassociated with the first method of the prior art and further avoids theexpensive regrinding operation associated with the second method.

In particular the method of the present invention provides for theformation of a ball screw member in a workpiece having a first end, asecond end opposite the first end, a longitudinal axis extending fromthe first end to the second end, a cylindrical portion interposed thefirst end and the second end, and a cylindrical outer surface on thecylindrical portion. The method includes two machining steps. One ofthese two machining steps provides for the machining of an externalhelical ball screw thread in the cylindrical outer surface, the externalhelical ball screw thread being located during such machining steprelative to the cylindrical outer surface. The external helical ballscrew thread has a partial circular cross-sectioned helical groove anddefines a helical crest surface adjacent thereto. The other of the twomachining steps provides for machining an end journal or other endtreatment at the first end of the workpiece, the end journal or otherend treatment being located during this other machining step relative tothe helical crest surface of the external helical ball screw thread ifthis step is performed after the machining of the external ball screwthread, or the external ball screw thread being located relative to theend journal or other end treatment during this other machining step ifit is performed before the machining of the ball screw thread.

In the preferred embodiment, the cylindrical outer surface is centerlessground before the above recited two machining steps are carried out.Furthermore, in the preferred embodiment, the workpiece is inductionhardened between the above recited two machining steps. If needed, thehelical crest surface may be centerless ground after the machining ofthe external helical ball thread.

A principal object of the present invention is to provide a method ofproducing a helical ball screw thread of a predetermined intermediateaccuracy on a ball screw member. Another object of the present inventionis to provide an inexpensive method of producing an external helicalball screw thread on a workpiece. In particular, it is an object of thepresent invention to provide a method of producing an external helicalball screw member of commercial or industrial quality and having aprecision intermediate that produced by the two methods of the prior artdescribed in the background. Furthermore, it is an object of the presentinvention to provide a method of producing an external helical ballscrew member of commercial or industrial quality which avoids the use oflocating pins or a regrinding step.

These and the many other objects, features and advantages of the presentinvention will become apparent to those skilled in the art when thefollowing detailed description of the preferred embodiment is read inconjunction with the drawings appended hereto.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a partially cutaway view of a portion of a ball screw assemblyof the prior art;

FIG. 2 is an enlarged partial sectional view through a ball screw memberformed according to a prior art method;

FIG. 3 is an enlarged partial sectional view through a ball screw memberformed according to the method of the present invention;

FIG. 4 is a plan view of a ball screw member according to the presentinvention; and

FIG. 5 is an end view of the ball screw member of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, and more particularly to FIGS. 1 and 2thereof, a typical ball screw assembly 10 and a typical ball screwmember 12 of the prior art are illustrated. The ball screw assembly 10and the ball screw member 12 are exemplary of those which may be madeaccording to the method of the present invention.

The ball screw member 12 according to the prior art is an elongatedmember having a first end 14 and a second end, not illustrated, disposedremote from the first end. A longitudinal axis 16 extends between thefirst end 14 and the second end.

The first end 14 of the ball screw member 12 is provided with suitableend treatments for cooperation with various machine elements. The ballscrew member 12 illustrated is provided with an end journal 18 having anend journal cylindrical surface 20 for rotatably mounting the ball screwmember 12. The end journal 18 may be provided with a first end surface22 normal to the longitudinal axis 16 as well as an aperture 24extending transversely through the end journal. The aperture 24 may beused for interconnecting the ball screw member 12 with other machineelements of the device, not illustrated, utilizing the ball screwassembly 10.

The ball screw member 12 is further provided with an enlargedcylindrical portion 26 interposed the first end 14 and the second end.An external helical ball screw thread 28 is formed on the enlargedcylindrical portion 26. As shown in FIG. 2, the external helical ballscrew thread includes at least one helical groove 30 having a partialcircular cross-section. At least one helical crest 32 is formed adjacentthe helical groove 30. As depicted in FIG. 1, a flat radial shoulder 34is formed between the enlarged cylindrical portion 26 of the ball screwmember 12 and the end journal cylindrical surface 20 thereof.

A nut 36, well known in the art and, therefore, not described herein inany further detail, is fitted over the enlarged cylindrical portion 26of the ball screw member 12. The nut 36 is provided with an internalhelical ball screw thread 38 complementing the external helical ballscrew thread 28 of the ball screw member 12. A plurality of bearingballs 40 are interposed the external helical ball screw thread 28 andthe internal helical ball screw thread 38 in a known manner.

Referring to FIG. 2, a detailed partial view of the external helicalball screw 28 of a ball screw member 12 made according to prior artmethods is illustrated.

When the ball screw member 12 is made according to a first method of theprior art, the external helical ball screw thread 28 is initially formedfrom the original cylindrical surface 42 of the enlarged cylindricalportion 26 by a rolling operation. The helical groove 30 formed by therolling operation typically has a tolerance of within twelve thousandthsof an inch (0.012 in) per linear foot.

The rolling operation produces a helical crest 32 having an irregularsurface, as indicated in FIG. 2. The helical crest 32 is, therefore, oflittle use in locating end treatments relative to the external helicalball screw thread 28 for subsequent machining operations. If forming andfinishing operations on the end treatments are located relative to thesurface defined by the helical crest 32, a substantial inaccuracy isintroduced by the irregularity of the surface. Furthermore, due to thehelix angle of the helical groove 30, the helical groove is difficult touse for locating and machining end treatments relative to the externalhelical ball screw thread 28. The use of locating pins to locate thefinishing operations on end treatments relative to the pitch diameter ofthe helical groove 30, as is well known, is more accurate than using thehelical crest 32 for locating end treatments but is difficult to performand to automate.

Some of the end treatments of the ball screw member 12 may be locatedrelative to other end treatments. For example, the end treatments suchas the first end surface 22, the aperture 24, and the flat radialshoulder 34 illustrated in FIG. 1, may be located relative to the endjournal cylindrical surface 20 after the end journal cylindrical surfacehas been located relative to the helical crest 32 or the helical groove30.

The first method of the prior art described above is used for theproduction of commercial or industrial quality ball screw members ofmoderate precision.

While the ball screw member 12 produced by the rolling operation iscomparatively inexpensive, the tolerance build-up between the helicalgroove 30 and the various end treatments is unacceptable for somepurposes. Therefore, according to an alternate method of the prior art,an additional grinding operation is typically performed on the helicalgroove 30 to reduce this tolerance build-up after at least onepreselected end treatment has been located. The grinding tool for theregrinding operation on the helical groove 30 is positioned relative tothe preselected end treatment of the ball screw member 12, thepreselected end treatment typically being the end journal cylindricalsurface 20. The regrinding operation substantially reduces the tolerancebuild-up between the end treatments and the helical groove 30 to withintwo to five ten thousandths of an inch (0.0002 to 0.0005 in.) per linearfoot.

While the second method of the prior art results in an extremelyaccurate ball screw member 12 of high precision, it is also extremelyexpensive to perform the regrinding operation. Therefore, the secondmethod of the prior art is only used when a high precision ball screwmember is needed.

Furthermore, the second method of the prior art still results in ahelical crest 32 of irregular shape. Therefore, a substantial gap mustbe provided between the helical crest 32 of the external helical ballscrew thread 28 and the corresponding helical crest 43 of the internalhelical ball screw of the nut in order to avoid engagement of portionsof the helical crest 32 with portions of the helical crest 43.

As will shortly be apparent to those skilled in the art, theseshortcomings of the first and second methods of the prior art, describedabove, are avoided by using the method of the present invention to forman external helical ball screw on a ball screw member.

According to the method of the present invention, a ball screw member112, shown in FIGS. 3, 4 and 5, is formed from a workpiece similar tothat used for the ball screw member 12 described above As shown in FIG.4, the ball screw member 112 has a first end 114 and a second end, notillustrated, disposed remote from the first end. A longitudinal axis 116extends from the first end 114 towards the second end of the ball screwmember 112. Certain end treatments are provided at the first end 114from the ball screw member, such as an end journal 118 having an endjournal cylindrical surface 120, a first end surface 122, and atransverse aperture 124.

The ball screw member 112 is further provided with an enlargedcylindrical portion 126 for the formation of an external helical ballscrew thread 128 having at least one helical groove 130 and a helicalcrest 132. Finally, a flat radial shoulder 134 is formed between theenlarged cylindrical portion 126 and the end journal cylindrical surface120.

According to the method of the present invention, and as best shown inFIG. 3, the original cylindrical surface 142, shown only schematically,of the enlarged cylindrical portion 126 of the ball screw member 112 ismachined to a predetermined tolerance by a centerless grinding operationto provide a new cylindrical surface 144. Next, one or more helicalgrooves 130 are machined in the new cylindrical surface 144. Themachining operation producing the helical groove 130 is positionedrelative to the new cylindrical surface 144. It should be noted thatwhen the helical groove 130 is machined, the helical crest 132 formedadjacent thereto will be substantially unaffected by the machiningoperation. Thus, the helical crest 132 will define a helical crestsurface 146 substantially identical to the new cylindrical surface 144described above. The helical groove 130 will be accurate to within threethousandths of an inch (0.003 in.) per linear foot. It should be notedthat this tolerance build-up is of an intermediate magnitude betweenthat which is experienced by the two prior art methods of forming thehelical groove 30 in the ball screw member 12. In practice, thisintermediate tolerance build-up is satisfactory for many applications.

Accordingly, the helical crest surface 146 may be used after the helicalgroove 130 has been machined so as to accurately position machiningtools used for final machining operations on the end treatments of theball screw member 112 or to locate the ball thread for remachining, ifnecessary. Alternatively, the end treatments of the ball screw member112 can be final machined before the helical groove 130 has beenmachined, and the final machined end treatments can be used toaccurately position machining tools used for machining the helicalgroove 130. Furthermore, if the machining operation used for forming thehelical groove 130 causes flaws in the helical crest surface 146, asecond centerless grinding operation may be performed on the helicalcrest surface 146 to clean up the helical crest surface, if necessary,so it may be reliably used for locating the end treatments relative tothe ball screw member 112.

According to the method of the present invention, the workpiece fromwhich the ball screw member 112 is formed is preferably inductionhardened after the helical groove 130 is machined. If desired, the endjournal 118 may also be induction hardened at this time. Alternatively,if the end treatments of the ball screw member 112 are final machinedbefore the helical groove 130 is machined, the workpiece from which theball screw member 112 is formed is preferably induction hardened beforethe helical groove is machined.

Next, the finishing operations may be performed on the various endtreatments of the ball screw member 112. The tools used for thefinishing operations on the end treatments of the ball screw member 112are accurately positioned relative to the helical crest surface 146. Thefinishing operations may include machining the end journal cylindricalsurface 120 to reduce the tolerance build-up between the end journalcylindrical surface and the external helical ball screw thread 128.

Other machining operations which may be applied to the end treatments ofthe ball screw member 112 relative to the helical crest surface 146 orthe new cylindrical surface 144 include drilling of the transverseaperture 124, and face grinding the first end surface 122 and the flatradial shoulder 134. It will be appreciated by those skilled in the artthat various other machine operations may be provided to other suitableend treatments for the ball screw member 112 at this time.

It will be appreciated by those skilled in the art that locating thesefinishing operations relative to the helical crest surface 146 is moreeasily accomplished than using locating pins or a regrinding operationand further that it may be easily automated. The method of the presentinvention provides a ball screw member 112 having a tolerance build-upbetween the helical groove 130 and the various end treatments of theball screw member which is intermediate those provided by the abovedescribed methods of the prior art. Nonetheless, the method of thepresent invention is less expensive to perform on a workpiece thaneither of the prior art methods. Thus, for those applications where atolerance in the range of a few thousandths of an inch is acceptable,the method of the present invention is highly desirable as a costsavings. Furthermore, the method of the present invention provides aball screw member 112 having a substantially improved positioningaccuracy, straightness, concentricity, and consistency of outer diameterthan the ball screw member 12 of the prior art produced by a rollingoperation alone, without a subsequent machining operation.

Furthermore, since the helical crest surface 146 produced according tothe method of the present invention is substantially straight and round,it may, if desired, be proportioned closer to the dimensions of thehelical crest 43; shown in FIG. 1, of the internal helical ball screwthread 38 of the nut 36 which is also used in conjunction with the ballscrew member 112 of FIGS. 3 through 5.

Finally, a high precision ball screw member may be formed from the ballscrew member 112 described above by performing a regrinding operation toregrind the helical groove 130 to a higher accuracy. The regrindingoperation may be located relative to the helical crest surface 146,resulting in a high precision ball screw member of greater accuracy thanthe high precision ball screw member of the prior art.

The above description constitutes the best mode contemplated at the timeof filing by the inventor for carrying out the present invention. Itwill be apparent to those skilled in the art that many variations andmodifications may be made therefrom without departing from the spirit ofthe present invention. Such variations and modifications are within theintended scope of the claims appended hereto.

What is claimed as novel is as follows:
 1. A method of forming a ballscrew member from a workpiece having a first end, a second end oppositesaid first end, a longitudinal axis extending from said first end tosaid second end, a cylindrical portion interposed said first end andsaid second end, and a cylindrical outer surface on said cylindricalportion, said method comprising:a first machining step of machining endtreatments on said workpiece adjacent said first end of said workpiece;and a second machining step of machining a helical ball screw thread insaid cylindrical outer surface, said helical ball screw thread beinglocated during said second machining step relative to said endtreatments, said helical ball screw thread having a partial circularcross-sectioned helical groove and a helical crest surface adjacentthereto.
 2. The method of claim 1 further comprising before said firstmachining step the additional step of centerless grinding saidcylindrical outer surface of said cylindrical portion of said workpiece.3. The method of claim 1 further comprising between said first andsecond machining steps the additional step of induction hardening saidcylindrical portion of said workpiece.
 4. The method of claim 1 furthercomprising, after said second machining step, a machining step ofcenterless grinding said helical crest surface of said helical ballscrew thread.
 5. The method of claim 1 further comprising between saidfirst and second machining steps the additional step of inductionhardening said cylindrical portion and said first end of said workpiece.6. The method of claim 1 further comprising after said second machiningstep the additional step of machining an end face on at least one end ofsaid first and second ends of said workpiece, said end face beinglocated during said additional machining step relative to said helicalball screw thread.
 7. The method of claim 1 wherein said end treatmentscomprise an end journal axially aligned with said helical ball screwthread.
 8. The method of claim 1 further comprising after said secondmachining step a step of regrinding said partial circularcross-sectioned helical groove relative to said helical crest surface.9. A method of forming a ball screw member from a workpiece, having afirst end, a second end opposite said first end, a longitudinal axisextending from said first end to said second end, a cylindrical portioninterposed said first end and said second end, and a cylindrical outersurface on said cylindrical portion, said method comprising:a firstmachining step of centerless grinding said cylindrical outer surface ofsaid cylindrical portion of said workpiece; a second machining step ofmachining an end journal at said first end of said workpiece; and athird machining step of machining a helical ball screw thread in saidcylindrical outer surface, said helical ball screw thread being locatedduring said third machining step relative to said end journal, saidhelical ball screw thread having a partial circular cross-sectionedhelical groove and a helical crest surface adjacent thereto.
 10. Themethod of claim 9 further comprising between said second and thirdmachining steps the additional step of induction hardening saidcylindrical portion of said workpiece.
 11. The method of claim 9 furthercomprising after said second machining step the additional step ofinduction hardening said cylindrical portion and said first end of saidworkpiece.
 12. The method of claim 9 further comprising after said thirdmachining step a machining step of centerless grinding said helicalcrest surface of said helical ball screw.
 13. The method of claim 9further comprising after said third machining step the additionalmachining step of machining an end face on at least one end of saidfirst and second ends of said workpiece.
 14. The method of claim 13wherein said end face is located during said additional machining steprelative to said helical crest surface.
 15. A method of forming a ballscrew member from a workpiece, having a first end, a second end oppositesaid first end, a longitudinal axis extending from said first end tosaid second end, a cylindrical portion interposed said first end andsaid second end, and a cylindrical outer surface on said cylindricalportion, said method comprising:a first machining step of centerlessgrinding said cylindrical outer surface of said cylindrical portion ofsaid workpiece; a second machining step of machining an end journalsurface at said first end of said workpiece; a third machining step ofmachining a helical ball screw thread in said cylindrical outer surface,said helical ball screw thread being located during said third machiningstep relative to said end journal surface, said helical ball screwthread having a partial circular cross-sectioned helical groove and ahelical crest surface adjacent thereto; a hardening step of inductionhardening said cylindrical portion of said workpiece; and a fourthmachining step of centerless grinding said helical crest surface of saidhelical ball groove.
 16. The method of claim 15 further comprising aftersaid fourth machining step a step of regrinding said partial circularcross-sectioned helical groove relative to said helical crest surface.